Can anybody decipher these numbers?

[Jacoby]

Trying to figure out what engine I have in my 68 C10. It’s a V8 I know that much lol. The plugs I pulled out of it work in anything from a I6 up to a 400 big block but don’t work in a 283, 307 or 396 which I believe were the only V8 options for 68. They do however come up as the go to plug for the 350. So this makes me think I got a transplant of some sort.

I tried searching these numbers and I think it’s a 350 but I’m not entirely convinced. Figured you guys could help me out. Thanks